1. Field of the Invention
The present invention relates to a capacitor; for example, a capacitor which is employed as a pulse generating capacitor (non-linear capacitor).
2. Background Art
Typical ordinal capacitors exhibit a linear characteristic. However, pulse-generating capacitors employed in an HID lamp, such as a high pressure sodium lamp or a metal halide lamp, have non-linear characteristics wherein the capacitance of the capacitor varies in a non-linear manner as a function of the applied voltage. These characteristics are achieved as a result of the materials, e.g., BaTiO3 and BaTi(Zr)O3, used for the ceramic body.
In typical pulse-generating capacitors, the electrodes are formed on the ceramic body using an electrode paste comprising Ag powder mixed with borosilicate glass or lead borosilicate glass, and varnish.
The glass is used because it enhances the bonding force between the electrode and the ceramic body. However, when electrodes are formed on a ceramic body by thermally applying such an electrode paste, the borosilicate glass or lead borosilicate glass diffuses into the ceramic body thereby degrading the pulse characteristics which are inherent to the ceramic substrate. Furthermore, the thickness or state of the resultant glass layer varies as a function of the method of printing or thermal conditions used to apply the electrode paste which induces variations in the nonlinear characteristics of the resultant capacitor.
Pulse-generating capacitors usually have lead terminals which are soldered onto the electrodes. Since they are used in the base of an HID lamp, the capacitors are usually subjected to high temperatures. However, when pulse-generating capacitors having lead terminals are used at high temperatures, the electric characteristics of the capacitors deteriorate. This is because Sn contained in the solder used to electrically connect the lead terminal to the electrode diffuses into the electrode, and the diffused Sn weakens the bonding between the glass and metal in the electrode or the bonding between the glass and the ceramic substrate. Therefore, an improved pulse-generating capacitor suitable for use at high temperature has been demanded.
In view of the foregoing, an object of the present invention is to provide a capacitor which exhibits consistent electric characteristics. Another object of the present invention is to provide a capacitor which comprises lead terminals soldered onto electrodes and which maintains its electric characteristics at high temperature.
In order to attain the aforementioned objects, the present invention provides a capacitor comprising a ceramic body, a glass layer formed on each of opposite surfaces of a ceramic body, and first metallic layer formed on the glass layer.
By virtue of the aforementioned structure, variations in the electric characteristics of the capacitor can be reduced, since the ceramic body is separated from the first metallic layers by the glass layers. When the ceramic exhibits a non-linear characteristic, the material of the glass layer is preferably lead bismuth borosilicate glass or barium borosilicate glass. This is because such glass does not readily diffuse into the ceramic body when electrodes are formed on the ceramic body through thermal application of the electrode, and thus pulse characteristics inherent to the ceramic body can be developed.
The glass layers are formed at high productivity by thermally treating an admixture of the ceramic body and glass powder while stirring the admixture, or by applying a paste containing metallic powder and glass powder onto the ceramic body and then firing the resultant body. Particularly, when the admixture of the ceramic body and glass powder is thermally treated while the admixture is stirred, the glass layers are formed to a uniform thickness on the surface of the ceramic body. As a result, variations in the electric characteristics of the capacitor are further reduced, and the withstand voltage of the capacitor is enhanced.
When the first metallic layers are formed of a barrier element xe2x80x98Mxe2x80x99 where Mxe2x80x94O bond energy is stronger than Snxe2x80x94O bond energy, they act as a barrier layer preventing Sn from migrating to the glass layer. Particularly, even if the capacitor is used at high temperatures and the Sn contained in the solder diffuses into the electrodes, the first metallic layers prevent diffusion of Sn and migration of Sn into the glass layers. Therefore, the electric characteristics of the capacitor do not deteriorate. Examples of the aforementioned barrier material include Ti, Mo, W, V, Cr, Ni, and alloys thereof.
Preferably, lead terminals are connected to the first metallic layers, or to second metallic layers which are provided on the first metallic layers, by use of solder containing Pb in an amount of 2.5 wt. % or less. Preferably, lead terminals which are plated with a substance containing Pb in an amount of 5 wt. % or less are soldered onto the first metallic layers or the second metallic layers.
As described above, since the amount of Pb contained in the solder or the amount of Pb contained in the substance which is employed for plating the lead terminals is low, the amount of Pb which diffuses within the electrode decreases, and Pb does not readily invade into the glass layers. Consequently, the electric characteristics of the capacitor tend not to deteriorate.